be up to no good
informalto be behaving in a dishonest or bad way:
He certainly looked as if he was up to no good.
Thesaurus: synonyms and related words
Dishonest
- bad faith
- be rotten to the core idiom
- bent
- cowboy
- crooked
- dubious
- false
- finagle
- fishy
- fly-by-night
- malfeasance
- mendacious
- misleading
- perjure
- rich
- underhand
- unreliable
- unscrupulous
- unsporting
- weasel
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You can also find related words, phrases, and synonyms in the topics:
Bad and wrong behaviour